In the past, video camera tubes have been designated "low velocity" or "high velocity". Low velocity tubes have typically featured better detective quantum efficiency and contrast than high velocity tubes. High velocity tubes have, on the other hand, typically featured better lag performance (less lag) and spatial resolution and permitted the use of larger capacity layers in the sensor-target than low velocity tubes. Such characteristics are discussed in (1) the Electronics Engineers' Handbook, Second Edition, edited by Donald G. Fink, 1982; (2) Television Camera Tubes: A Research Review by P. K. Weimer in Advances in Electronics & Electron Physics, Vol. XIII, pp. 387, Academic Press, New York and London, 1960 (3) Photoelectronic Imaging Devices, Edited by L. M. Biberman and S. Nudelman, Plenum Press, New York, 1971; 4. Electronic Image Storage by B. Kazan and M. Knoll with contributions by Wittarth, Academic Press, New York and London, 1967; 5. The High Beam Velocity Vidicon by J. Dresner R.C.A. Review, 305, June 1961; and 6. Advances in Image Pickup & Display, Ed. by B. Kazan, Volumes I & II, 1974, 1975 Academic Press, New York and London. The desirable features of both the low velocity tube and of the high velocity tube have not, until the present invention, been incorporated into a single tube. It should be noted that the low velocity tube currently is the only tube used in practice due to its higher efficiency and contrast.
Two classes of scanner image tubes are currently in use. The first operates "without" gain and is exemplified by the vidicon, Plumbicon, Chalnicon, Saticon, Newvicon and Silicon vidicon. The second operates "with" gain and is used in situations where there is insufficient light to operate "without" gain. These tubes are exemplified by the image orthicon, SEC and SIT tubes. The unique feature of this latter group is the incorporation of a front end intensifier structure designed to provide image charge multiplication. In both classes of tubes, it is observed that there is included an imaging section and an electron beam scanning section. It should be noted, however, that in the "no gain" tube, the imaging section comprises simply a disc-like layer of material. It serves the dual function of being a photon sensor and a target on which to store a layer of electronic charge. For this reason, it is referred to herein as the sensor-target.
In a "low velocity," no gain tube, the electron beam scans the inside surface of the sensor-target in a raster scanning fashion. It scans adjacent parallel lines of the sensor-target one after another. The electrons arrive at the sensor-target surface with low energies, and in particular, too low to cause any secondary electron emission. In the process, it deposits electrons uniformly across the surface and drives it to approximately the potential of the electron gun chathode which is usually at or near ground. Secondly, the electron beam generates a time varying video signal as it scans through the raster. This results from modification of the charge on the sensor target as a result of projection of the optical image onto the input sensor surface of the image tube. The process occurs on a successive pixel by pixel basis as electrons lost on the target surface through image formation are replaced by the scanning electron beam.
The imaging section incorporates a medium with two functions; a sensor responsive to the incident radiation to be imaged and an insulating target having a resistivity on the order of 10.sup.12 ohm cm. The high resistivity is essential to maintaining electron charge storage and immobility on the inner scanned surface of the sensor-target during a raster scanning period. On exposure of the sensor-target to incident radiation, from an image of an object, there results a flow of charge through the medium such that electrons are lost from the scanned surface. Accordingly, electrons are lost across the surface in numbers proportional to the changing intensity of irradiation comprising the incident image. The resultant electronic image is "readout" by the scanning electron beam.
The tubes "with gain" have a forefront intensifier type structure which separates the sensor from the target. As a result, the sensor changes from a vidicon photoconductor to a photoelectron emitter.
In operation, photoelectrons generated from the sensor's absorption of incident irradiation are accelerated in the tube vacuum by the intensifiers electron optics and are imaged onto the target's outer surface. The electrons strike that surface with sufficient energy to cause charge multiplication and a loss of a proportionate number of electrons stored on the targets inner surface. As a result, the scanning electron beam must replace more stored electrons per absorbed photon than in the "no gain" tube, and the video signal is amplified.
The scanner image tubes and the prior art have experienced various problems, especially where large target area is required. Conventional low velocity tubes in such applications suffer when the high beam resistance is coupled to the large capacity to result in undesirable excessive lag. Large capacitance can result from increasing the size of the target, increasing the mediums dielectric constant and/or decreasing its thickness.
In general, numerous factors must be considered if the scanner image tube is to perform effectively and optimally. Besides the sensor-target's capacity and distributed capacity, other factors include the energy of the electron beam, its resistance and current, as well as the modulation transfer function (MTF) of the tube's imaging components. The latter depends on such factors as target thickness, the lateral displacement of charge stored on the sensor-target inner surface during a raster period, and beam current discharge characteristics.
Moreover, a high detective quantum efficiency (DQE) of preferably 100% is a desired feature for an optimal scanner image tube. A DQE of 100% corresponds to a sensor having a quantum efficiency of 100%; output noise limited by the photon noise at the input where the image is projected initially--i.e. photon noise, should dominate other electronic sources of noise; and the MTF high enough to assure that the dynamic range matches the imaging requirement throughout the necessary spatial frequency spectrum.
For large area X-ray imaging, conventional, low velocity beam scanner tubes have failed to meet the needs of diagnostic radiology. As an alternative approach, X-ray intensifiers have been developed with diameters up to 22 inches. Such intensifiers are than optically coupled to a conventional sized TV camera. These systems are used in fluoroscopy and diagnosis. They suffer from intrinsically poor spatial resolution compared to that of the X-ray sensor. This results from multiplication of component MTF's. Poor spatial resolution manifests itself medically in two ways, i.e., in reduced diagnostic performance and increased dose to the patient. These deficiencies must be overcome.
The operation of a "high velocity" beam video tube differs from the "low velocity" beam in that the electrons of the scanning beam are made to strike the inner surface of the sensor-target with sufficient energy to cause secondary electron emission. In an ideal tube, the electron bombarded surface gives off secondary electrons, which are collected by "collecting" electrodes. In this process the scanned surface becomes positively charged because of the lost secondary electrons, and the surface voltage rises until at equilibrium, the scanning beam is essentially equivalent to the collected beam.
When the sensor is then exposed to incident light there is a loss of positive charge in a manner that causes an electronic image reproduction of the incident irradiation image. The scanning beam then is able to replace the positive charge lost through secondary electron emission until the equilibrium potential is reached. In the process of replacing lost charge, it generates a corresponding video signal on a pixel-by-pixel basis which effectively creates the time varying video signal that is similar to that from the "low velocity" tube.
The Iconoscope, in the United States, and its counterpart, the Emitron, in Great Britain, were the first video tubes to incorporate the features of "high velocity" scan and the principle of charge storage.
The Iconoscope, however, suffered from the low sensitivity and poor efficiency resulting in large part from poor collection and redistribution effects of secondary electrons.
A "high velocity" tube with a photoconductive sensor target was demonstrated in experimental tubes in the early 1950's. The purpose was to develop a new concept which would overcome the problems of lag and limited sensitivity. It was discovered that under certain conditions of operation, the tube could be made to operate with a high capacity target and provide improved lag performance. Furthermore, the higher capacity (thinner) targets also offered superior spatial resolution, and in. theory, were expected to provide superior quantum efficiency. Redistribution effects were reduced in that the photoemitted electrons did not exist. The secondary electrons generated during scanning were less of a problem in one mode of operation and worse in the other. Shading continued to be bothersome, but again on a reduced scale due to better collector design. Nevertheless, the approach was dropped as the low velocity vidicon-type tubes were improved to the point where the met industrial and broadcast requirements.
However, applications exist that require the use of large capacity targets. These cannot be carried out with the conventional "low velocity vidicons" where target capacity and stray capacity must be limited. The "high velocity" approach can serve as the basis for a new invention which offers the opportunity to provide a new imager able to combine the best features of the "high and low" velocity tubes.
Diagnostic radiology, for example, requires large area imaging with attendant sensor-target capacity and distributed capacity both prohibitively large with "state of the art" video tubes.
Solid state sensor panels employed in X-ray imaging are under development but suffer from other disadvantages. Some such sensor panels provide that readout be performed on the same side of the panel that is exposed. Moreover, the panel undergoes a cycling of voltages to respectively effectuate charging, reading, writing, erasing, and recharging. Accordingly, the panel must be transported from one station to another, rendering rapid imaging impossible. When such a system is automated, a large mechanical transporter is incorporated to move the panel from the exposure platform to its readout station. Disadvantages include cost, space occupied by the system, and the time involved to complete the process.